This invention relates to photopolymerizable resist film and more particularly to such film in which improved release properties can be obtained.
U.S. Pat. No. 3,469,982 discloses a negative acting photopolymerizable film resist in which a photopolymerizable layer is sandwiched between a temporary support film and a temporary cover film. This film has gained widespread usage in the manufacture of printed circuits in which the cover film is removed, the photopolymerizable layer is laminated by heat and pressure to the surface, e.g., copper, of a substrate to be permanently modified, the layer is imagewise exposed to actinic radiation, the film support is removed, the unexposed areas of the layer are removed by solvent washout (development), and the resultant bared area of the copper surface is permanently modified, such as by etching or deposition of metal.
The patent discloses solution coating of a photopolymerizable composition onto the support film, followed by drying and then lamination of the resultant dry layer to a surface such as copper-clad printed circuit board to carry out the photoresist process. In commercial practice, however, the dried photopolymerizable layer, which must remain adhered to the support during storage and shipment, has always been supplied as a sandwich between the support film and a cover sheet. This has enabled the sandwich element to be rolled up upon itself by the photoresist manufacturer and be supplied to the user, e.g., printed circuit manufacturer, as a compact, easy to handle roll. The cover film permits the roll to be unrolled by the user without the cover film pressing the prior art photopolymerizable layer against the backside of the support film and causing that layer to adhere. In use, the cover film is stripped away and discharged, followed by the photoresist processing described above.
This type of photopolymerizable film sandwiched between a temporary support film and a temporary cover film can be formulated to be useful in solder mask applications wherein the photopolymerizable film is applied to portions of a printed circuit board surface to confine molten solder heated to a temperature of at least 230.degree. C. to pad areas on the board and to prevent bridging between conductors during tinning operations and during soldering of components. A solder mask also functions to prevent or minimize corrosion of the bare copper conductors and as a dielectric to insulate certain components from adjacent circuitry. Since a solder mask remains on the finished circuit board, i.e., the solder mask is a permanent coating for the printed circuit, it usually is transparent to allow for ease of circuit inspection.
Prior to use of the photopolymerizable layer it is necessary to remove the cover sheet to allow the photopolymerizable layer to be applied to a substrate which may or may not contain circuitry.
The present invention is directed to the introduction of an additive into a negative acting photosensitive composition which will allow easier removal of the cover sheet.
Use of additives in photosensitive layers is known in the prior art. Additives useful in the present invention are known in U.S. Pat. No. 3,827,908 to improve adhesion of positive photoresist to oxide semiconductor surfaces.